1. Field of Invention
The present invention generally relates to analysis of solutions. More particularly, the present invention relates to online automated matrix elimination and trace contaminant analysis of process chemical solutions.
2. Discussion of the Related Art
Chemical solutions are used in various manufacturing processes in a multitude of industries, including the semiconductor, environmental, and pharmaceutical industries. A solution sample includes a matrix, defined herein as a liquid solution, suspension, or colloid, and may or may not include a detectable amount of at least one analyte of interest. Examples of matrixes are diluted or concentrated acids, bases, oxidants, reducing reagents, solvents (such as alcohols, esters, ethers, glycols, ketones, amides, amines, or their mixtures), cleaning solutions, photoresists, strippers, and developers. Examples of analytes of interest are metals and their species.
The matrix of a solution sample has a pronounced effect on the quantification of trace constituents by modern analytical instruments. For example, a common problem is detecting analytes of interest in a matrix including one or more compounds of high ionic strength. Many times, the desired analyte peaks or signals are obscured by the large interfering peak of a matrix ion. In many analytical instruments, the detector is saturated with matrix ion signals and is not able to distinguish the desired analyte signal.
In some cases, the desired signal is suppressed because the matrix ions compete with the desired analyte ions when ionization of the sample occurs for analysis purposes. For example, during ion formation in electrospray ionization, the matrix ions can solvate or deprotonate the analyte of interest resulting in less ion formation of the analyte of interest.
The composition or properties of a matrix may also change from process to process and during the life of the sample, which may then affect the recovery of an analyte from a complex matrix. Analyte speciation may further compound this effect. The stability of a sample/analyte may also change during analysis because of a changing thermal regime or photolytic effect. Thus, inaccurate analysis of a sample may occur because of the transitory nature of the matrix.
In many instances, however, accurately monitoring the analytes of a sample at a specific point in time during a process is highly desirable. For example, in semiconductor manufacturing processes, monitoring the metallic impurities in a cleaning solution is of immense importance for producing reliable devices with high yield. Purity of these solutions during offline and online processes is very important as well.
Reliably measuring the metallic contaminants in a cleaning solution at a parts-per-trillion level, especially in the presence of a high matrix, is not only complicated but also laborious and time consuming. For example, a solution of hydrochloric acid (HCl), hydrogen peroxide (H2O2), and water in varying ratio may be used to remove the metallic residues from the surface of a silica wafer by forming chloride complexes and dissolving in the solution. The solution is commonly known as a Standard Clean 2 solution (SC2). The most common ratio for SC2 used in semiconductor manufacturing is one part of 37% HCl to one part of 30% H2O2 to six parts of ultra pure water (UPW).
The continuous decrease in the geometry of devices requires increased control of the contaminants in a solution such as SC2. Control over the contaminants is important because SC2 comes in direct contact with the electronic circuitry during device fabrication. Thus, the quantitative determination of metallic contaminants in fresh and spent SC2 solution is of immense importance, for example in determining the cause of a defect or the endpoint of a process.
Due to the high matrix of chloride ions in SC2, the simultaneous online determination of trace levels of many metals is very difficult. In the absence of an analytical instrument that can monitor online all the contamination levels of metals in SC2, it is common practice to collect the SC2 sample before, during, and/or after a process of interest and to send the samples to an offline analytical laboratory for analysis. Typically, it takes between 4 to 24 hours before the analysis results are received by process personnel. Accordingly, in most cases, if a problem is detected, such as impurities in the SC2, processing of defective product will have occurred for some time and the cost related to low yield will be high.
Another problem with offline analysis is maintaining the integrity of the SC2 sample starting from collection to the end of analysis. First, SC2 cleaning is typically done at elevated temperatures, between about 60° C. to about 75° C., and at this temperature the matrix of SC2 is dynamic in nature such that the components of the SC2 are continually reacting with other components and can change over time. Thus, by the time the sample reaches a laboratory for analysis, the sample may not be in a representative formulation as it was at the time of collection. Second, the SC2 matrix is a strong absorption media for airborne soluble contaminants such that if samples are exposed to air at any stage during sampling, transportation, or analysis, the matrix of the sample may be altered or contaminated. Third, the cleanliness of the sampling containers is important and a large amount of time and money is spent on cleaning sampling containers. The amount of time the sample is allowed to sit in the sampling container before being analyzed can also affect the analysis outcome. It has been reported that even the cleanest of sampling containers can leach out many undesirable contaminants. Fourth, offline elimination, neutralization, or modification of matrixes generally poses a high risk of contamination that can affect the integrity of the sample for the reasons stated above.
Depending on the nature and concentration of the SC2 matrix, various analytical laboratories have developed their own methods to test a sample including an SC2 matrix. For example, some laboratories dilute the sample to reduce the effect of the matrix but by doing so many ultra low trace level contaminants may not be detected. Other laboratories eliminate the matrix by heat and/or evaporation but by doing so potentially lose the integrity of the sample constituents.
As a result, many semiconductor device manufacturers are in need of online measurements to provide substantially real-time analysis. Therefore, a method and apparatus for accurate online elemental and molecular analysis of process solutions, such as SC2, on a substantially real-time basis are highly desirable.